Contrasting Thermodynamics Governs the Interaction of 3-Hydroxyflavone with the N-Isoform and B-Isoform of Human Serum Albumin

Abstract

Herein we report the interaction of 3-hydroxyflavone (3HF) with various isomeric forms of Human Serum Albumin (HSA), namely, the N-isoform (or native HSA at pH 7.4) and the B-isoform (at pH 9.2). Spectroscopic signatures of 3HF reveal that the interaction of 3HF with the N-isoform of HSA results in significant lowering of absorbance of the neutral species (λabs ∼ 345 nm) with concomitant increase of the anionic species (λabs ∼ 416 nm) whereas interaction with the B-isoform of HSA leads to selective enhancement of absorbance of the anionic species. The fluorescence profile of 3HF displays marked increase of intensity of the proton transferred tautomer (λem ∼ 538 nm) as well as the anionic species (λem ∼ 501 nm) for both the forms of the protein. However, analyses of the associated thermodynamics through temperature-dependent isothermal titration calorimetric (ITC) indicate that the interaction of 3HF with the N-isoform of HSA is more enthalpic in the lower temperature limit while the entropy contribution predominates in the higher temperature limit. Consequently, the 3HF–HSA (N-isoform at pH 7.4) interaction reveals an unusual thermodynamic signature of a positive heat capacity change (ΔC p = 3.84 kJ mol–1K–1) suggesting the instrumental role of hydrophobic hydration. On the contrary, the 3HF–HSA (B-isoform at pH 9.2) interaction shows qualitatively reverse effect. Consequently, the interaction is found to be characterized by an enthalpy-dominated hydrophobic effect (negative heat capacity change, ΔC p = −1.15 kJ mol–1K–1) which is rationalized on the basis of the nonclassical hydrophobic effect.

Exploring the Nucleobase-Specific Hydrophobic Interaction of Cryptolepine Hydrate with RNA and Its Subsequent Sequestration

“…This is an in-built nature of the instrument. However, the fitting parameters obtained from the software provide the actual thermodynamic signatures of the concerned process, and we analyzed our data accordingly. , …”

“…However, the fitting parameters obtained from the software provide the actual thermodynamic signatures of the concerned process, and we analyzed our data accordingly. 21,22 ■ RESULTS AND DISCUSSION Photophysics of Cryptolepine Hydrate. The prototropic equilibrium between the protonated and the neutral forms of CRYP accounts for pH-sensitive photophysics of the drug.…”

“…48 Burial of nonpolar surfaces inside the nucleobases drastically reorganizes the structure of water molecules of hydration and thus loosens the water−water interaction in the hydration shell of the nonpolar surfaces, lowering the heat capacity value as a result of the interaction. 21,24,48 This phenomenon facilitates reorgan-ization of the water molecules around the nonpolar surfaces, which is the major contributor to the negative heat capacity change associated with a hydrophobic interaction. 21,24 We also studied the interaction of G and A nucleobases with CRYP at 296 K (Figure S8), and the thermodynamic parameters are summarized in Table 2.…”

“…21,24,48 This phenomenon facilitates reorgan-ization of the water molecules around the nonpolar surfaces, which is the major contributor to the negative heat capacity change associated with a hydrophobic interaction. 21,24 We also studied the interaction of G and A nucleobases with CRYP at 296 K (Figure S8), and the thermodynamic parameters are summarized in Table 2. A negative enthalpy change (ΔH < 0) suggests that the interaction of G and A nucleobases with CRYP is exothermic in nature, and these interactions are also entropically favorable (TΔS> 0).…”

Interaction mechanism of collagen peptides with four phenolic compounds in the ethanol-water solution

“…4e. A reaction between CPs and phenolic compounds obviously occurred, and it was an exothermic process because of the downward heat-burst curves presented [38].…”